I took a test a couple days after I should have started my period. I saw a faint line too. I waited a week and took another...sure enough I was pregnant. So most likely u are just wait another week to be sure.
Yes it's possible to be pregnant after one time. And unless you were using OPK's you really don't know when for sure you ovulated, either way 4 days is close to ovulation. You can try a test with first morning urine, if there's even a 2nd line at all, you're pregnant. Good luck hun!